(1.) Field of the Invention
The present invention relates to an image forming apparatus which forms images on the recording medium by causing the developer to jump thereto and can be applied to a printer unit in digital copiers and facsimile machines as well as to digital printers, plotters, etc.
(2.) Description of the Prior Art
In recent years, as the image forming means for outputting a visual image on recording medium such as recording paper etc., in response to an image signal, there have been various technologies such as Japanese Patent Application Laid-Open Hei 7 No. 178,953 as well as Japanese Patent Application Laid-Open Hei 9 No. 193443 which was filed by the present applicant. In these disclosures, image forming apparatuses have been proposed in which charged particles are placed in an electric field so that they will jump by electric force to adhere to the recording medium whilst the potential to be applied to the control electrode having a number of passage holes located in the jump path is being varied, to thereby form an image on the recording medium, directly. In the above techniques of the prior art, the control electrode has a single driver configuration, and the application time of voltage and the toner discriminating method suited to image forming are described.
In a type of image forming apparatus of the above prior art, the passage of the charged particles through the gates is controlled in accordance with the data of an image so as to produce dots of the image data on a sheet of paper as the recording medium thus creating the image. In this process, in order to allow the toner to pass through gates and reach the paper thereby forming dots on it, a voltage is applied continuously for a predetermined period of time. This continuous period of time for voltage application is long enough to make the toner reach the control electrode. Then, the potential of the control electrode is switched to a voltage at which no toner will jump immediately after the toner has passed by the control electrode. This control of timing makes it possible to achieve improved speed of printing.
In the technology disclosed in Japanese Patent Application Laid-Open Hei 7 No. 178,953, the toner unsuitable for image forming is removed from the toner support before the toner arrives at the position facing the gates. This configuration needs additional removing means such as a roller, power source etc. Besides, in some cases, the toner may present different characteristics between the time when the toner faces the removing means and when the toner faces the gates. Resultantly, it is not the case that only toner having the correct characteristics will be used for image forming.
In the technology disclosed in Japanese Patent Application Laid-Open Hei 9 No. 193443, the potential enabling toner to jump continues to be applied until all the toner involved has passed by the control electrode. From the view point of the printing time, the voltage causing the toner to jump must continuously be applied until the toner has passed by the control electrode. Accordingly, this imposes a limit on reducing the duration of voltage application per dot, resulting in difficulty in increasing the printing speed further still.
If the voltage to be applied to cause the toner to jump (to be referred to as the ON potential) is enhanced in order to shorten the duration of voltage application, it is possible to reduce the time until all the toner involved has passed by the control electrode, but an increased amount of toner might jump, possibly heightening the toner density more than is required. Resultantly, the desired dots fail to be formed making it difficult to reproduce correct halftones. In order to obtain a desired amount of toner that jumps, the size of gates can conceivably be reduced. In this case, however, since large amounts of toner pass through gates at a time, the gates are liable to become clogged, quickly causing image defects and print deficiencies.